Department of Materials Science and Engineering, University of Science and Technology Beijing, Haidian District, Beijing 100083, China.
Chemphyschem. 2012 Dec 7;13(17):3812-8. doi: 10.1002/cphc.201200729. Epub 2012 Oct 12.
The photocontrolled phase transitions and reflection behaviors of a smectic liquid crystal, 4-octyl-4'-cyanobiphenyl (8CB), tuned by a chiral azobenzene, are systematically investigated. For the smectic 8CB doped with the chiral azobenzene (1R)-(-)-4-n-hexyl-4'-menthylazobenzene (ABE), the initial smectic phase can be switched to cholesteric and then to isotropic upon UV irradiation due to the trans-to-cis photoisomerization of ABE; however, no reflection band is observed. For the smectic 8CB doped with ABE and the chiral agent (S)-(-)-1,1'-binaphthyl-2,2'-diol (BN), a reflection band located in the short-wavelength infrared region is observed, which disappears after further UV irradiation. For the smectic 8CB doped with ABE and a chiral agent with higher helical twisting power, (S)-2,2'-methylendioxy-1,1'-binaphthalene (DBN), a phototunable system with cholesteric pitch short enough to reflect visible light is demonstrated. With a given concentration of the chiral dopant DBN, a reversible reflection color transition is realized tuned by the isomerization of azobenzene. The reverse phase transition from isotropic to cholesteric and then to smectic can be recovered upon visible irradiation. The photocontrolled phase transitions in smectic liquid crystals and the corresponding changes in reflection band switched by photoisomerization of azobenzene may provide impetus for their practical application in optical memories, displays, and switches.
手性偶氮苯调控的近晶相液晶的光控相转变和反射行为。研究了手性偶氮苯（1R）-（-）-4-正己基-4'-薄荷基偶氮苯（ABE）掺杂的近晶相 8CB 的光控相转变和反射行为。由于 ABE 的顺式-反式光异构化，初始近晶相可以光切换到胆甾相，然后光切换到各向同性相；然而，没有观察到反射带。对于掺杂 ABE 和手性剂（S）-（-）-1,1'-联萘-2,2'-二醇（BN）的近晶相 8CB，观察到位于短波长红外区域的反射带，进一步的紫外光照射后反射带消失。对于掺杂 ABE 和具有更高螺旋扭曲能力的手性剂（S）-2,2'-亚甲基二氧基-1,1'-联萘（DBN）的近晶相 8CB，展示了一个光可调谐的胆甾相，其螺距短到足以反射可见光。在给定浓度的手性掺杂剂 DBN 的情况下，通过偶氮苯的异构化实现了可逆的反射颜色转变。在可见光照射下，从各向同性相到胆甾相再到近晶相的逆相转变可以恢复。近晶相液晶的光控相转变和偶氮苯光异构化引起的反射带变化可能为它们在光存储、显示和开关等实际应用中提供动力。
